I added warnings to the overlay last week to inform people that it was meant for development only. I take it that you saw them.

With that said, this issue on github has shown me that the the population of users using the 9999 ebuilds is bigger than I originally thought. There was an issue with the 9999 ebuilds where it is easy for sys-fs/zfs-kmod to be rebuilt without rebuilding sys-fs/zfs after a new feature changed the kernel/userspace interface. This could cause issues because the interface could become out-of-sync. I have committed an update to the main tree that makes sys-fs/zfs-9999 add itself to the module database. It should be on the mirrors within a few hours. Anyone using the sys-fs/zfs-9999 ebuild should rebuild sys-kernel/spl-9999, sys-fs/zfs-kmod-9999 and sys-fs/zfs-9999 to ensure that sys-fs/zfs-9999 is both in the module database and it is in sync with sys-fs/zfs-kmod-9999 and sys-kernel/spl-9999.

Note that sys-fs/zfs-9999 will not add itself to the module database when built with USE=kernel-builtin.